I've alway figured that the bridge railing was never meant to be at safety height for the upper stations. It was designed to be at Kirk-lean-upon height from below.
But take a look at the railings in Wrath of Khan: Standing on the lower level, Kirk is able to rest his chin on them while declaring that he "feels young." There you have a functional guard rail. It's about the only feature of the movie-era bridge that I like better than TOS.